Rubber and plastic articles (hereinafter abbreviated to "articles") are widely used as automobile and bicycle parts, various industrial articles, building materials, and so forth. In these fields, the articles are used as unexpanded or expanded materials or combinations of these materials, or combinations thereof with different materials such as metals. One of important characteristics required for such articles is that the articles have good surface properties and stable shape.
The surface properties of the article referred to herein include not only properties concerning external appearance such as surface smoothness, luster, and vivid color but also mechanical properties such as surface hardness, wear resistance, scratch resistance, and low friction resistance as well as chemical properties such as oil resistance and organic solvent resistance.
These properties are important for the article regardless of whether the article is expanded or unexpanded. In particular, they are important when the article is in the expanded form (which is also called "a sponge article"). That is, in the case of such an expanded article, unevenness is liable to develop in the surface thereof during the production thereof. As a result, a beautiful surface having smoothness and luster can be hardly obtained. Also, if blowing agents or decomposition products thereof go out through the surface from the molding during the process of expansion, water absorbing resistance and wear resistance are lost and at the same time, soft touch cannot be obtained.
Another problem is that non-crosslinked rubber and plastic moldings cannot retain their desired forms during the process of production thereof because of their own weight, deformation and distortion.
In the industrial production of stabilized article surface, the production of expanded articles is more difficult than the produciton of unexpanded articles. This is considered due to the fact that the production of expanded articles requires addtionally an unstable step of exapansion.